[User Error] Bad class to singleton() - ConfigPage? GET /?flush=1

Hello! Sorry for my english, I'm from Belarus. I created its type of the page "ConfigPage?", as it was described here: http://webxadmin.net/silverstripe-tutorials/. I have understood, what create its own check and I have wanted to delete type of the page "ConfigPage?". When I have deleted file mysite/code/ConfigPage.php and has tried to execute request /db/build?flush=1, I have seen this error: